Variable capacity device and circuit



June 3 194,1- N. M. RUsT Erm. 2,243,921

VARIABLE CAPACITY DEVICE AND CIRCUITS Filed Jan. 9, 1940 5 Sheets-Sheet 1 Il xd W31 lIIIFIIIIIIIII IIIl Gttomeg June 3, 1941. N. M. RUST r.AL 2,243,921

VARIABLE CAPAUITY DEVICEAND CIRCUITS Filed Jan. 9, 1940 5 Sheets-Sheet 2 S OURCE j+4, mw

CIttorneg June 3, 1941.

N. M. Rus-r Erm. 2,243,921

VARIABLE CAPACITY DEVICE AND CIRCUITS Filed Jan. 9, 1940 5 sheets-sheet s F 0. TcL /70 In Vc F/L TER Gttomeg vJune 3, 1941. N. M. RUST rAL 2,243,921

VARIABLE CAPACITY DEVICE AND CIRCUITS Filed Jan. 9, 1940 5 Sheets-Sheet 4 Ffgflz: Jd l? uz l CII

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Gttorneg June 3, 1941. N. M. RUST ETAL. 2,243,921

VARIABLE CAPACITY DEVICE AND CIRCUITS Filed Jan. 9, 1940 5 Sheets-Sheet 5 (Ittorneg Patented June 3, 1941 UNITED STATES PATENT OFFICE VARIABLE CAPACITY DEVICE AND CIRCUIT Application January 9, 1940, Serial No. 313,149 In Great Britain November 12, 1938 (Cl. Z50-40) 2 Claims.

This invention relates to variable capacity arrangements and has for its object to provide improved and simplified apparatus wherein a variable capacity effect controllable by an applied variable E. M. F. may be obtained.

Though not limited to its application thereto the invention is particularly well suited for the obtaining of automatically variable frequency selective filters which, though not limited in application thereto are primarily intended for, and particularly suitable for use in, broadcast and other radio receivers where they may be employed to provide automatic selectivity control and/or automatic frequency control, and for the obtaining of automatically variable audio correction circuits which though not limited in application thereto are primarily intended for, and particularly suitable for use in broadcast and other radio receivers where they may be employed to provide automatic tone control and/or automatic audio disturbance elimination.

Research conducted by Messrs. J ohnsen and Rahbeck and published in the I. E. E. Journal for July 1923 (vol. 61, No. 320) has shown that when a conductive foil or similar conductor is placed in close juxtaposition to a semi-conductor such as a piece of lithographie stone, agate, or the like, and a voltage is applied between the semi-conductor and the foil or the like a very substantial adhesion which varies with the applied voltage takes place between the foil or the like and the semi-conductor if the surfaces in contact are suitably polished. This effect has become known as the Johnsen-Rahbeck effect and it has been proposed to employ the said effect for Various electro-mechanical conversion purposes.

The present invention in its broadest aspect uses the J ohnsen-Rahbeck effect and effects similar thereto, to produce a variable capacity which may be controlled by an applied voltage.

According to this invention in its broadest aspect a variable capacity effect controlled by an applied voltage is obtained by means including a conductive body having a polished insulating surface film incorporated therein or deposited thereon in close contact with either (1) another conductive body also having a similar surface film or (2) a conductor provided with a polished surface, vand electrode means which, in the case of a conductor in contact with the polished surface of the conductor may include the said conductor, are provided so as to enable the potential across the polished contacting surfaces to be controlled. Variable voltage is applied between the conductors provided for that purpose and it has been found that variation of this voltage produces a substantial variation in the capacity between one of the conductors and another, fixed, conductor in electrostatic association therewith.

According to a feature of this invention, a radio or like receiver is provided with an A. F. C. (automatic frequency control) system comprising a potential variable condenser of the nature above described incorporated in or coupled to the circuit whose tuning is to be controlled and operated by A. F. C. potential obtained from a discriminator and superimposed on a suitable polarizing voltage.

According to a second feature of this invention an automatically variable frequency selective filter comprises at least one element constituted by a condenser in accordance with the invention, and the said condenser is varied by a varying applied voltage which is obtained in dependence upon the type of control required, said condenser, by varying its value, serving to alter the characteristics of the filter in the particular manner required.

An important application of this feature of the invention is to the obtaining of automatic selectivty control in a radio receiver. When so applied the invention may be utilised to give control of such form that the acceptance band of the receiver is automatically moved away from a nearby interfering signal or, should there be two nearby interfering signals, one on each side of the normal acceptance band, the said acceptance band is automatically narrowed.

According to a third feature of this invention an automatically variable audio correction circuit comprises at least one element constituted by a condenser in accordance with the invention and said condenser is varied by a varying applied voltage which is obtained in dependence upon the type of correction, said condenser, by varying its Value, serving to alter the audio response characteristics of the circuit in the particular manner required. An important application of the invention is to the obtaining of automatic tone control in a radio receiver. Another application is to the obtaining of automatic elimination of heterodyne whistle interference in such receivers.

In the drawings- Fig. 1 shows a variable condenser embodying the invention,

Fig. 2 shows a modified form of condenser,

Fig. 3 shows still another form of condenser construction,

Fig. l shows the invention applied to an A. F. C. system of a superheterodyne receiver,

Fig. 5 illustrates the method of controlling C of Fig. l as applied to a remote-controlled receiver ci the push-button type,

Fig. 6 shows the invention applied to an automatic selectivity control system,

Fig. 7 shows the operating characteristics of the circuit of Fig. 6,

Fig. 8 shows a modification of the I. F. band control of Fig. 6,

Fig. 9 shows a modified type of automatic selectivity control employing the invention,

Fig. l shows a modiiication of Fig. 9,

Fig. 1l shows the operating characteristics of the circuit oi Fig. i0,

Fig. l2 shows another modification of the system oi Fi 6,

Fig. 13 shows schematically a receiver employing audio tone control,

Fig. le shows the tone control circuit used in i3 with connection t1, and

Fig. l shows the tone control circuit used with connection te,

Figs. 16 and 17 show two diierent types of interference reduction circuits embodying the present invention.

in one yay of carrying out the invention and illustrated in Figure l of the accompanying drawings, a variable condenser comprises a piece of slate S having one surface curved and highly polished; a piece of copper foil C cemented or otherwise suitably fixed to a ilat face of the slate; a piece or polished copper foil C in intimate contact with the curved surface of the slate; a iixed auxiliary electrode A spaced from the polish d copper foil C" and co-operating therewith to constitute the electrodes oi the condenser; and means, represented by the source B and resistance R, for applying a variable potential between the two pieces of foil. With this arrangement it has been found that the spacing and tl'iereore the capacity between the two electrodes which constitute the electrodes of the condenser varies with the applied potential the variation being due to movement oi the polished ioil (as shown by the dotted line) with respect to the curved surface oi" the slate.

Although the above embodiment is operable and does produce a variable` condenser effect, it is not preferred because there occurs a substantial hysteresis effect. As the Voltage is increased the capacity decreases but on subsequent decrease of the voltage the capacity variation does not correspond to the variation during increase oi' voltage but, on the other hand, the capacity remains fairly constant as the voltage is reduced and then suddenly increases rapidly with further decrease in voltage. Owing to this hysteresis eiiect the arrangement just described will be found unsuitable for many purposes and in general will not be i'ound suitable for use (in manner to be described later herein) for A. F. C. purposes.

in another embodiment oi this invention and illustrated in Figure 2 oi the accompanying drawings, two pieces of agate or slate AS and AS-preierably agateare employed and each is provided with an accurately prepared plane surface The two plane surfaces are placed in contact with one another and cemented on the remaining parallel :dat faces of the pieces ci agate or slate are pieces of metal foil MF and MF (one on each) so that the two pieces of metal foil are parallel to one another with the two pieces of slate or agate between them. Variable potential is applied between the metal foils. With this arrangement it has been found that with a suitable voltage applied-lor example a voltage of 500 or so-the capacity varies very considerably with variation of applied potential. This embodiment is considerably better than the rst described embodiment from the point of View ci hysteresis effect, but the power factor of the condenser is rather poor and it is diilicult to produce the same results time and time again.

In the preferred embodiments or" this invention the di-electric of a variable condenser in accordance therewith is constituted by a very thin insulating lm on a metal surface. For example in one preie 1red embodiment of this feature of the invention one of the electrodes of a condenser consists ol a plate ol' aluminium which is carefully surfaced and 'hen provided with a very thin film oi insulating material, i'or example a film of collodion of the order of 0.001 mm. thick or less. The other electrode of the condenser is constituted by a foil or other suitable conductor in Contact with the nlm. A polarising voltage ol' 2G() volts, or thereabouts is applied between the :toil and the plate-i. e. the electrodes of the condenser. A substantial and reproducible variation in capacity with varie ation in polarising voltage is obtained, there being little hysteresis eiiect. Similar good results can be obtained by employing, in place or co1- lodion coated aluminium, an aluminium electrode which is chemically oxidised to provide a thin nlm.. Again and as illustrated in Figure 3 of the accompanying drawings, an aluminium electrode AE with an anodised surface may be employed, the other foil electrode being designated F. T and T are terminals one of which passes through and makes contact 'with AE and the other of which with a bent strip of foil ST makes contact with F. MES and MS" are mica spacers and MW is a nuca washer, EC is an ebonite cover and EB a base. The whole assembly is held together by a screw passing through holes in the elements, the hole in F lbeing somewhat larger than the outside diameter of MW. A suitable thickness for the spacers and washers is 0.002. Although it is not so suitable as the arrangement o1 Figure 3, good results can be obtained from` a condenser consisting of two-faced, anodised, aluminium plates with the anodised surfaces in Contact.

The actual physical action taking place in variable condenser in accordance with this invention is somewhat obscure, but it is believed that the phenomena utilised depend upon big attractive eiiects which arc set up across very thin di-electric films.

A condenser in accordance with this invention and such that it is comparatively free from hysteresis eiect, is very suitable for incorporation an A. F. C. system for a radio or like receiverfor example, for a push button operated superhetercdyne receiver. ln push button operated superheterodyne receivers it is almost essential to provide a measure of A. F. C. in order to meet difficulties due to inaccurate tuning arising from slight errors of manufacture (in the case of a push button receiver with mechanically operating tuning gear) and variations in electrical constants (in the case oi a push button receiver with purely electrical tuning apparatus). Simplifiment above described and illustrated in Figure 8 of the accompanying drawings, the two series circuits SC and SC which are in parallel with one another in the above described embodiment are not connected directly to the junction point of the two main inductances of the filter but are connected thereto through a third series resonant circuit SC tuned to the mid-band frequency. A single variable condenser VC in accordance with the invention is included in a circuit across the condenser in this third rejector circuit SC and is subjected to a voltage composed of a predetermined rixed bias component and a variable component derived from. a double diode resistance network RE fed from two filters as hereinbefore described. In this circuit, the net voltage across RE. constitutes the variable component. With this arrangement, an increase in the value of the variable condenser VC will have the effect or sliding the band pass characteristic in the low frequency direction while a decrease will have the effect of sliding it in the high frequency direction.

With fairly slight modiiication the two embodiments above described may be adapted to produce automatic frequency control as distinct from automatic selectivity control. For example, in the second embodiment, the iilters which feed the rectier circuits from which in turn the variable condenser is operated may be so arranged that on the departure of the produced I. F. from the niid-band frequency of the I. F. amplier, the variable condenser changes value in a direction to alter the tuning of the coupling iilter so as to accord with the said produced I. F. automatic frequency. Thus A. F. C. is obtained without variation of the local oscillator o'f the receiver. ln other words, in this manner it is possible to obtain A. C. action in a superheterodyne receiver by automatically varying the effective tuning of the I. circuit. This is, of course, in contradistinction to present day practice in A. F. C. which has been to maintain the I. F. tuning fixed and to control the local oscillator frequency to make the produced I. accord with the predetermined I. F. Among the advantages of the method of the present invention is that it avoids the ganging errors so frequently encountered in known A. E'. C. systems due to the heterodyne oscillator going out ci adjustment.

In another embodiment of the invention and illustrated in Figure 9 of the accompanying drawings, a coupling lter provided between two I. F. stages is automatically controlled dependence upon the voltage derived from a known A. V. C. (automatic volume control) system so as to control the selectivity in relation to signal strength thus to provi-de high delity reception .for strong signals and high selectivity reception for weali signals. This iilter comprises a pair of parallel tuned circuits TC and TC having parts of the inductances thereofI coupled inductively to one another. Connected across the tuning condenser in one tuned circuit is a circuit comprising a variable condenser VC and a second similar variable condenser VC is similarly included in a circuit connected across the corresponding tuning condenser in the other tuned circuit. Equal and opposite biases are applied to the two variable condensers and the total voltage across each of these condensers is caused to be the algebraic sum of the bias potential and a potential derived from the A. V. C. system. In the absence of control voltage from the A. V. C. system the two tuned circuits in the filter have their coupling adjusted just below the optimum coupling at which singie peak response is obtained. If the A. V. C. Voltage increases the voltage across one variable condenser is increased and that across the other decreased with the result that the resonant frequencies of the two coupled tuned circuits are staggered in opposite directions and a broad response curve with reduced gain is obtained.

The invention is not limited to the particular lter circuits described. For example another lter of variable selectivity and illustrated in Figure l0 of the accompanying drawings, comprises a pair of main inductances connected together at one end and a pair of condensers each connected at one end to the end of one or other of the main inductances, the other ends of said condenser being connected through a further condenser. C to the junction point of the inductances. The elements thus far enumerated constitute tuned circuits TC and TC. Yet another condenser C is connected between taps (which may be variable) on the two main inductances and a variable condenser VC in accordance with the invention hereinbefore referred to is included in circuit across this last mentioned condenser a similar variable condenser VC being likewise included in a circuit across the condenser which is connected to the junction point of the two main inductances. Equal and opposite polarising potentials are applied from a source represented by resistance R to the two variable condensers, the total voltage across each condenser being made up of a permanent bias component together with a control voltage obtained in any of the ways already described and app-lied by way of terminals T and With this arrangement, increase of one of the variable condensers will cause a widening of the band pass in a direction away to one side of the mid-band frequency (see curves l, 2, 3, of Figure ll) while increase of the other variable condenser will cause a similar widening of the band pass in the other direction, By providing an inductance in parallel with the condenser C between the tapping points and also providing another inductance in series with the condenser C which is connected at one end to the common point of the two main inductances so that the two circuits including the voltage variable condensers VC and VC are respectively of the parallel resonant and series resonant types, it is possible to obtain a larger range of variation with a similar range of capacity variation than would otherwise be the case.

It is not necessary in automatic selectivity control circuits, to provide separate discriminator circuits to provide the voltage for operating the variable condenser or condensers. For example, and as shown in Figure l2, in an I. F. filter designed for a superheterodyne receiver for a predetermined I. E. of, say, 450 kc. there might be employed in the iilter two dip circuits, one resonant at 440 lrc. and the other at 460 kc. Said filter could comprise in one line and in the order stated two inductances LI and LZ, a condenser C and two more inductances Ll and L2, the junction points ,between the pairs of the inductances being connected to the other line through further condensers C and C. The two ends of said one line are connected to the other line (in each case) through a condenser and the two inductances forming each pair are so arranged that there is positive mutual coupling between them. Automatic control of such a lter could be obtained by connecting a diode D' and D across each` of 'the condensers C" and C connected to thel induct'ance junction points, and superimposing the rectified voltages from these diodes uponl equal and opposite bias potentials applied from 'ai-source represented by resistance R to two voltage variable condensers VC and VC", one of these variableccndensers being included ina circuit across one and the other of these variable 'condensers being included in a circuit across the `other of the two condensers C and C-' Onelembodiment of the third feature of this invention wherein automatic tone control of a Isuperheterodyne receiver is obtained in such manner as to increase the higher audio irequency response'when strong signals are received and to decrease it when weak signals are received, will now be described in connection with Figures 13` and14 of the accompanying drawings. In this embodiment the receiver comprises the usual pre-selector stage l, local oscillator and first detectorf or converter 2, intermediate frequency amplifier 3- and demodulating detector 4. Between the demodulating detector 4 and the usual low frequency amplifier 6 and associated loudspeaker is afvariable low pass low frequency filter L. F. The receiver is fitted with the usual A. V. C-. system l operating, for example, from an intermediatel frequency amplifier stage, and effective to -control the gain of one or more of the preceding parts of the receiver for anti-fading purposes asin the usual Way.

In accordance with this invention unidirectional-output voltage from-the A. V. C. unit is also applied to vcontrol the low pass low frequency iil'ter L.F. so as to cause the upper limiting frequency of said filter to rise in response to a largeA. V. C. output `and to fall in response to a small one. he low pass filter may be of anyknown general design, for example as shown in Figure 14 it may comprise a number of inductancesV L', L and L" in series in one line and a number of sh nt condensers each connected between the other line and one of the inductancev terminals, but, in accordance with this invention, these condensers are variable condensers VC', VC", VC and VC". A suitable predetermined polarising voltage is applied from a source B through a high resistance R to all these condensersand there is also app'iied to said condensers, output voltage derived from the A. V. C. unit ('lv of Figure 13), the arrangement being such that increase in negative A. V. C. output volts reduces the capacities of the variable con.- densers inthe iilter, thereby raising the cut-off frequency,

In a modification illustrated in Figure 13 bythe removal ofstrap'SR from terminal tl to t8, the cut oflffrequency oi the low pass filter is controlled,` independence upon the amount of high audio frequency present, in such manner as to'lowerthe said cut-off frequency in response to an increase in high audio frequency. As will be appreciated a vgreat part of the noise and interference in a radio receiver is of high audio frequency and therefore. this embodiment enables the best compromise to be automatically obtained between the conflicting requirements of maximum fidelity and minimum noise and interference. In this embodiment the A. V. C. unit is not employed to control the filter but is merely utilised in the ordinary well known way for antifading gain control. Instead the low pass filter L. F. is controlled in dependence upon the energy passed by a high pass filter II. F. which is fed from the output from demodulating detector 4. For example as shown in Figure 15 the high pass filter H. F. may consi-st of a number of series condensers in one line and a nrunber of shunt inductances .to the other line and the output of this iilter may be fed tc a diode or other rectifier circuit RC to produce a unidirectional voltage which, together wtih a predetermined polarising voltage from source B is applied to the variable condensers in the low pass iilter, this low pass filter being as in the iirst described embodiment. The arrangement is such that increase in high audio frequency in the demodulating detector output produces an increase in the values of the variable condensers in the low pass filter and thereby a reduction in the cut-oh frequency of the low pass filter.

In yet another modiiication illustrated in Figure 13 chain line and by the removal of strap SR from terminal tl, or 'L8 to t9, interference by signals in an adjacent channel is minimised by controlling the cut-ofi frequency of the low pass filter in dependence upon the amount of adjacent channel energy present. In this embodiment any suitable convenient iilter or discriminator circuit DC, for example that illustrated in Figure 6, fed from a convenient intermediate point in the receiver, as shown, between 3 and 4, is adjusted to pass energy corresponding to interfering adjacent channel signals. The voltage thus passed is rectiiied and the resultant applied together with a `polarising voltage to the voltage variable condensers VC to VC" of a low pass lter (such as shown in Figures 14 and l5) connected and arranged as hereinbefore ascribed, the arrangement being such that the cut @if frequency of the low pass frequency is lowered in response to an increase in the output from the discriminator.

In yet another embodiment of the invention illustrated in Figure 16 of the accompanying drawings, provision is made for giving what may be termed automatic contrast amplification. If high audio frequency interference increases, one way of compensating for the loss of intelligibility which would ordinarily result therefrom is to increase the volume of the low notes and the present embodiment accomplishes this result. In this embodiment there connected across the grid input circuit or a low frequency Valve V for example the low frequency valve which feeds the usual loudspeaker LS, a circuit consisting of a resistance R, a voltage variabie condenser VC, a further resistance R and a polarising voltage source B all in serios in the order stated, the polarising voltage source being nearest the cathode of the valve and a blocking condenser and grid leal: being inserted in the usual way. The anode of the valve is resistance capacity coupled to the grid circuit of an additional isolator valve V whose anode receives anode potential through a circuit XC which mag7 be in the form of a circuit adapted to respond to high audio frequencies or may be in the form of a high pass filter. The anode of the isolator valve is coupled to a diode-resistance circuit DR, and the rectiiied voltage which appears across the resistance of this circuit is fed through any suitable further resistance R" to the grid side of the voltage variable condenser VC'. It will be seen that with this arrangement any increase in high audio interference will produce an increased gain at low audio frequencies thereby increasing the intelligibility to compensate for the increased high audio interference.

Another very common form of interference which :occurs in radio receivers is that known as heterodyne whistle i. e. a more or less persistent `audio frequency note or Whistle due to the interaction of the desired modulated carrier with an adjacent channel modulated carrier. As is well known this common form of interference manifests itself as :a disturbing note which is, of course, not always of the same frequency but may vary from time to time, and from station to station. This variation makes ordinary tuned lters very difficult, if not wholly impractical, to apply for the purposes of eliminating the interference since `a constant frequency sharply tuned rejector circuit is n'ot sufficient.

The present invention may be applied to obtain reduction or elimination of interference of this nature by providing `a rejecting filter which autof matically adjusts itself as may be required to the note to be rejected.

One embodiment of this nature as applied to a resistance-capacity coupled low frequency amplier will now be described in connection with Figure 17 of the accompanying drawings. In this embodiment of the invention the anode end of the usual ranode resistance of one low frequency valve V is coupled to the grid end of the usual grid resistance `of the next low frequency valve V through three condensers C', C, C" in series and :a resistance R is connected across those two of the three condensers C and C" which are nearer the grid of the second valve V. The junction point of these two condensers is connected to the cathode end of the grid resistance of the second valve V through a series resonant circuit SR comprising an inductance and a condenser in series. The cathodes `of the two valves :are each connected to earth through the usual capacity shunted bias resistance. The inductance of the series resonant circuit has, in parallel therewith, `'a condenser C in series with a diode D', `and a second diode D is connected across the condenser of the series resonant circuit. The two diodes have their cathodes connected together and each is in Iparallel with a load resistance RE and RE".

n The `diode cathode point-Which is of course electrically the same as the junction point `of the two-load resistances-is connected through a condenser to one side of a voltage variable condenser VC and the other side of this voltage variable condenser is connected to the cathode end lof the grid resistance of the second valve through a further condenser. Said lother side `of the variable condenser is also connected through a resistance R (of, for example 10 megohms) to the anode of the diode which is in parallel with the inductance in the series resonant circuit, while the first mentioned side of the said voltage vari-able condenser is connected through a source of negative potential (not shown) to earth. As will be appreciated the coupling circuit between the two low frequency valves is of the so-called resistance cancelled bridge-T type, and by suitable choice of the component values, the arrangement may be ma de to give a very low loss, the cross arm of the bridge-T throwing negative resistance into the; series resonant circuit thus decreasing its losses: and giving a sharp `absorption effect :at the't'unedl rejector frequency. In effect the voltage var-r iable condenser is included in the series reso-- nant circuit, The arrangement is made suchl that the series resonant circuit frequency is ncrmally about 9 kc. change in transmitter frequency or frequencies;- the interference or whistle note becomes higher than 9 kc. the voltage across the inductance part in the series resonant circuit will rise relative to that across the condenser therein; the voltage on the voltage variable condenser will decrease; the capacity of the said voltage vai'- iable condenser will decrease; and the tuning'. of the series resonant circuit will, therefore, alter in the required direction to approach very closely the whistle frequency. Should the Whistle frequency become lower an opposite effect takes place.

It will be appreciated that the feature` of the. last described embodiment is not limited tothe elimination of this particular type of disturbance in `audio frequency circuits, nor indeed to, 4audio frequency circuits at all, for the samer principle, namely the utilisation of a condenser' automatically to vary the resonant frequency of a series resonant circuit in accordance with ther frequency of energy fed thereto, may be appliedi for the elimination of any form of disturbaISce occurring in a narrow frequency band. In other words, the embodiments just described is a specic but by no means a limiting example of a self-tuning lter the self-tuning action being obtained by phase change brought about in the lter itself by a change in the frequency fed thereto.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. A device for obtaining a variable capacity effect by an applied voltage and including a first electrode comprising a piece of slate have ing one surface curved and highly polished; a piece of copper foil secured to a flat face of the siate; a piece of polished copper foil in inti mate contact with the curved surface of the slate; a Xed `auxiliary electrode normally spaced from the polished copper foil and providing the second electrode of the condenser; and means for applying `a variable potential between the two pieces of foil.

2. A device for producing a variable capacity effect in response to an adjustable potential comprising a first electrode which includes at least one plate of mineral insulator, la metallic foil secured to one face of the insulator, `a second metallic foil secured to the opposite face of the plate, a second electrode consisting of a metallic sheet spaced from said rst electrode, :and means for varying the potential difference between said foils.

Noam MEYER RUST.

JOSEPH DOUGLAS BRAILSFORD. ARTHUR LEONARD OLIVER. JOHN FORREST RAMSAY.

If, for example, owing to a: 

